iterator? way of generating all possible combinations?

[Scott David Daniels]

akameswaran at gmail.com wrote:
> Scott David Daniels wrote:
>> Sorry, "re-iterables".  A file re-iterable is:
>>
>>      class FileReIterable(object): ...
>>          def __iter__(self):
>>              self.file.seek(0)
>>              return iter(self.file)
>>
>> This works if-and-only-if it is only in use once at a time.
>> If you have multiple simultaneous accesses, you need to do
>> something like:
>>
>>      class FileReIterable2(object): ...
>>          def __iter__(self):
>>              self.file.seek(0)
>>              for line in self.file:
>>                  nextpos = self.file.tell()
>>                  yield line
>>                  self.file.seek(nextpos)
> 
> Since I was doing this as a self education excercise.  When you say is
> in use once and only once, you mean I can only use a single instance of
> the class?  
No.  This works:

     f1 = FileReIterable("one.file")
     f2 = FileReIterable("another.file")
     ... free uses of ...

This does not "work":

     gen = FileReIterable("one.file")

     for a in gen:
         for b in gen:
             print a, b

This does "work":

     gen = FileReIterable2("one.file")

     for a in gen:
         for b in gen:
             print a, b

That is, any instance of FileReIterable must not be used in a
context where it may be in the midst of iterating a file, gets
used to produce a result, and then must produce a result from
the original iteration.

If you think about what must happen to the file read pointer,
the reason for all of this should become clear.

--Scott David Daniels
scott.daniels at acm.org